1 How many stars can you see on a clear night? About 5,000 stars are visible from Earth without the use of a telescope, but you can only see a fraction of them from any particular viewing point at any particular time. With the help of a powerful telescope, it's possible to see more than 3 billion of the hundreds of billions of stars in our Milky Way Galaxy.2 Stars are huge masses of hot, glowing gases, made up of about 75% hydrogen, 22% helium, and traces of oxygen, neon, carbon, nitrogen, and other elements. Hydrogen provides the fuel for stars to release heat, light, and other forms of energy through a process called nuclear fusion. When the center of a star reaches a temperature of 2,000,000 degrees Fahrenheit, hydrogen gas is changed to helium gas. The extra hydrogen that is not used during nuclear fusion is instead changed into energy. This energy is transferred to the surface of the star and then passed into space in the form of light, heat, and radio waves. This release of energy is what causes stars to "shine."3 The combination of elements and amounts of each one found in a star is described as its chemical composition. As the chemical composition of a star changes over time, its appearance is also altered. For example, a star that is in the late stages of its existence may have exhausted its supply of hydrogen fuel, causing it to burn helium and heavier elements. This causes the star to expand greatly, transforming it into what is called a red giant. As the red giant loses material on its surface, gravity causes the star to begin to collapse. As the star's pressure and temperature increase, the star may explode into a nova (a star that suddenly becomes extremely bright). The star may then eventually become a white dwarf, which is the last visible stage of a star. If the star continues to cool down and contract, it may become a collapsed star called a black hole. Only very large, massive stars can become black holes.